1. Field of the Invention
The present invention relates to a thermal head of a thermal printer used as the printer for word processor or typewriter, or in the facsimile receiver.
2. Description of the Prior Art
A typical prior art is shown in FIG. 1. A thermal head 200 is basically composed of a head substrate 202 and a head cover 203 covering this head substrate 202. The head substrate 202 possesses a row of multiple resistance heating elements 204 forming heating dots on a substrate 212, and an external terminal 205 for giving electric signals for selectively heating the resistance heating elements 204. On the external terminal 205 of the head substrate 202, a terminal 207a formed on a wiring conductor 207 of a wiring substrate 206 is overlaid, and the both are mounted on a support board 208. The head substrate 202 and wiring board 206 are held and pressed by the head cover 203 and support board 208. A thermal recording paper 209 is fed in a direction vertical to the row of arrangement of the resistance heating elements 204 (the direction of arrow A). When this recording paper 209 is supplied, the heating resistance elements 204 are selectively energized and heated, and dots are formed on the recording paper 209 pressed by a platen 210 so as to be printed.
The resistance heating elements 204 are divided into groups in each plurality, and electrodes 211 connected to the resistance heating elements 204 are connected to an integrated circuit device 214 of each group disposed on the substrate 212. To the integrated circuit device 214, an electric signal is applied from an external electric circuit through the external terminal 205 formed on the substrate 212. This integrated circuit device 214 is covered with silicone rubber 215 excellent in water resistance and chemical resistance in order to protect the integrated circuit device 214 and its connection part. The leading end of this wiring conductor 207 is the terminal 207a.
The wiring substrate 206 has the wiring conductor 207 printed and formed on the surface opposite the head substrate 202 of the film 216 made of synthetic resin having an electric insulating performance. The external terminal 205 and terminal 207a are joined together, and are pressed by a pressing member 217 fixed to the head cover 203. The pressing member 217 is made of a synthetic resin possessing an elasticity, and is stretched over the whole length of the region in which terminals 205, 207a are disposed.
In the head cover 203, an insertion hole 218 is formed, and another insertion hole 219 is formed in the wiring substrate 206. In these insertion holes 218, 219, a bolt 220 penetrates loosely. A screw hole 221 is machined in a support board 208, and the bolt 220 is driven into this screw hole 221. Accordingly, the terminal 207a of the wiring substrate 206 and the terminal 204 of the head substrate 202 are overlaid on the support board 208, and when the head cover 203 is fixed to the support board 208 by the bolt 220, the both terminals 205, 207a are pressed and connected by the head cover 203 and support board 208.
When printing, in particular when printing continuously, if the temperature of the head substrate 202 and support board 208 is raised, the head substrate 202 may be warped due to difference in the coefficient of thermal expansion of them. In other words, since the coefficient of thermal expansion of the aluminum-made support board 208 is larger than the coefficient of thermal expansion of the ceramic-made substrate 212, when these temperatures are raised, the boards are curved within the plane extending in the arrangement direction of the resistance heating elements 204 (the direction vertical to the sheet of paper in FIG. 1), which is schematically shown in FIG. 2.
When such thermal head 200 is used in the facsimile equipment and the head substrate 202 is curved as in the above case, if the maximum displacement d1 due to warping of the head substrate 202 becomes more than 0.1 mm, the resistance heating elements 204 may partly fail to contact with the thermal recording paper 209, and the thermal transmission from the resistance heating elements 204 is worsened, so that printing may be unclear.